One way to deactivate your account is to simply stop signing in. If you don't want any future mails from CC, disable reminder mails in your settings. Then your account will automatically be deactivated after you haven't logged on in a year or so (note that if you have a recurring premium subscription you will need to suspend on your subscription page).
If you want to close your account immediately and permanently you can do so here.
If you want to remove your stories simply delete them yourself before quitting. Critiques on deleted stories can only be viewed by the person who wrote the critique.